Human basophils and mast cells develop from CD34-expressing progenitor cells in the presence of IL-3. Mast cells differentiate to mature phenotypes (as determined by appearance of certain protease isozymes and ultrastructural morphology) when cultured over 3T3 fibroblasts. The ligand of the human c-kit protooncogene (stem cell factor; mast cell growth factor) together with IL-3 enhances the proliferation of all cell lineages including basophils and mast cells and contributes to the maturation of human mast cells. Murine mast cells and also basophils develop from murine bone marrow in the presence of IL-3. However, IL-3 is the only obligatory growth factor for murine mast cells but does not function as a growth factor. Mast cells are found in diverse anatomical locations and adhere to surfaces coated with laminin or fibronectin. This process is enhanced during IgE-mediated activation. Attachment to laminin depends on a 5 amino acid (IKVAV) sequence, and to fibronectin by the RGD sequence. Murine mast cells also express ICAM-1 on their surface. FCepsilonRI crosslinking results in a genetic induction of many developmentally important genes including transcription factors and pro-inflammatory and growth factor cytokine genes which are differentially regulated. IL-6 is synthesized in vivo in mouse skin after mast cell degranulation. A neuropeptide (substance P) and thrombin induce pro-inflammatory cytokines in mast cells without histamine release. Murine mast cells secrete endothelin and also have endothelin receptors.